Lennie accidentally kills the puppy by petting or squeezing it too hard. Lennie's dream is to take care of rabbits. When the puppy dies, Lennie has no real remorse for the dog- as he's only worried about what George is going to say about the dead dog (and whether or not he'll let Lennie get rabbits). Lennie tries to justify it by saying that dogs aren't rabbits and that it was the dog's fault that it died in the first place. He even considers hiding the body.
From the definition of absolute phrases we know that it is a modifier that attaches to a sentence without a conjunctution. So, we can say "his chubby legs, pumping" is the correct answer.
